1. Field of the Invention
This invention relates to electrical devices that comprise film type electrical circuits. More particularly, this invention relates to a method and apparatus for the placement of an electrically conductive member, such as a wire, in electrical communication with a film device using ultrasonic welding means, and the application of this process to produce, for example, a heater.
2. Summary of the Prior Art
Various articles and methods of forming electrical connections are well known including soldering, spring loaded contact pads, welding or the use of electrically conductive epoxy adhesives. There are, however, problems in forming a suitably reliable electrical connection between a wire conductor and a terminal pad which is deposited on a substrate such as metal, steel, or ceramic, or a substrate comprising one or more metal, steel or ceramic components or layers that are placed in high temperature environments. Unfortunately, most of these well known articles and methods for attaching a wire have an upper temperature limit beyond which the original physical characteristics become unstable so that electrical connections so formed are incapable of reliably maintaining their attachment integrity over a wide temperature range.
For example, specialized solder is effective up to 280xc2x0 C. at which point the solder will melt and allow the wire to disconnect from the pad. In addition, the application and processing of specialized solder is time consuming. Silver epoxy is also used but silver-loaded epoxy is effective up to 250xc2x0 C. and is expensive and time consuming to process. The use of glass-loaded epoxy extends the temperature range up to 400xc2x0 C. but is very expensive and time consuming also. In extremely high temperature environments, spring loaded contacts have been employed but they are very complicated, time consuming and have a limited operating life. In addition the integrity of the spring contact reduces with age due to thermal cycling and accelerated oxidation. Other more conventional methods of attachment such as an electric arc or flame-burner welding have also been employed, but these are not adaptable for thick film terminal pads on a ceramic or ceramic based or coated substrate.
U.S. Pat. No. 6,039,238 to Panaghe discloses a method of attaching a conductor to a thick film trace by applying pressure to a terminal lug that is affixed to the conductor and ultrasonically welding the terminal lug to the film trace. However, this method is problematic and does not address every installation scenario. For example, a means for applying this method to a non-flat (eg., cylindrical) substrate is not addressed or contemplated. Further, this method also requires the attachment of a terminal lug to the proximal end of the conductor/wire which can be expensive and problematic. Yet another shortcoming of this invention is the requirement that the thick film pad be substantially the same thickness as the terminal lug. This would require either the use of a very thin terminal lug or a very thick thick-film pad.
The present invention has been provided to address these well known but heretofore unsolved problems associated with the connection of a conductor directly to a thick film pad for the communication of power thereto, particularly in a high temperature environment.
There is generally provided a method and apparatus for securing a lead wire in electrical communication with a terminal pad wherein said terminal pad is a thick film trace disposed preferably on a non-flat (ie. cylindrical or the like), substrate using ultrasonic welding techniques that provides a reliable and cost effective connection means in generally high temperature environments. The proximal end of the lead wire is densified and does not require the attachment of a terminal lug and hence the strand(s) of the wire are directly attached to the film pad for reduced cost and increased reliability.
It should be noted that while the foregoing specification will discuss heaters for use with injection molding machines, the invention is not so limited, and all embodiments of the disclosed invention are fully contemplated herein.
In accordance with a first general embodiment of the present invention, an injection molding nozzle in an injection molding machine is provided comprising an elongated injection nozzle for the communication of a molten material to a mold cavity, a heater in thermal communication with said injection nozzle, said heater comprising a substrate with a resistive trace thereon and at least a pair of thick film pads, a plurality of densified electrical conductors each connected to a respective thick film pad, an ultrasonic weld connecting said each densified electrical connector to the respective thick film pad.
In accordance with another general embodiment of the present invention, a method for the attachment of an electrical conductor to a thick film pad is provided comprising the steps of providing a substrate with said thick film pad disposed thereon, densifying said conductor and placing said conductor on said thick film pad, attaching said conductor to said thick film pad by applying ultrasonic energy to one of said conductor and said thick film pad.
According to another general embodiment of the present invention, an electrical conductor (eg. wire) is affixed to a thick film pad that has been disposed on, for example, (but without limitation) a cylindrical heater for use in the injection molding of plastic resin, by an ultrasonic welding process.
The thick film pad is in electrical communication with the electrical conductor and a thick film resistive trace. The thick-film resistive trace generally generates ohmic losses when current flows therethrough. The resistive trace is preferably applied to the surface of an electrically insulative and thermally conductive substrate such that heat generated by the resistive trace is communicated to the substrate and in turn to the article to be heated. In this general embodiment, there is no terminal lug attached to the end of the conductor but rather the individual strand(s) are attached to the thick film pad.
In accordance with yet another general embodiment in accordance with the present invention, a method for the attachment of an electrical conductor to a thick film pad is provided comprising the steps of providing a non-flat substrate with said thick film pad disposed thereon, densifying said conductor and placing said conductor on said thick film pad, attaching said conductor to said thick film pad by applying ultrasonic energy to said thick film pad.
In another embodiment, a cylindrical heater is provided that is comprised of a thermally conductive substrate which is in thermal communication with a body to be heated, for example, an injection molding nozzle. The heater can be comprised of a multi-layer thick-film applied on the surface of the cylindrical substrate wherein a pair of conductors is ultrasonically welded to a respective pair of thick film pads for the application of electrical power to the heater. The conductors are preferably made from wire that has been densified at their exposed ends prior to attachment to the thick film pad.